Energetics of the electron transfer from bacteriopheophytin to ubiquinone in the photosynthetic reaction center for Rhodopseudomonas viridis: Theoretical study
The energetics of electron transfer in the photosynthetic reaction center of Rhodopseudomonas viridis was studied using the density functional theory (DFT). By examining the basis set-dependence and the accuracy of the DFT for calculating adiabatic electron affinity, single-point calculations with 6-31+G(d) basis sets, at the geometry optimized with 6-31G(d) basis sets, were found to be almost independent of the basis set. In gas-phase calculations, bacteriopheophytin (H) had the greatest electron affinity among the three chromophores: H, menaquinone (MQ), and ubiquinone (UQ). However, the order of the electron affinity was reversed to be UQ > MQ > H by including residues that interacted with the chromophores through hydrogen bonding. Based on the QM/MM optimized geometries, cluster models for the binding sites were constructed. The computed reaction energy was comparable to values obtained experimentally. The reaction energy can be decomposed into a vertical electron affinity term and a relaxation energy term using a driving force analysis. The most important term was the vertical electron affinity of the chromophores. Based on optimization, there was little structural reorganization. The present results indicate that, with regard to the energetics of electron transfer, local interactions between the chromophores and proteins play a decisive role by tuning the electron affinity of the chromophores, whereas the effects of distant residues are of secondary importance.
Hasegawa, JY; Ishida, M; Nakatsuji, H; Lu, Z; Liu, H; Yang, W
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